As current typical LCD, STN (super twisted nematic) LCD and TFT (thin film transistor) LCD have existed. While STNLCD has a relatively low cost, the number of drive lines thereof is at most 500. The TFTLCD is also expensive to manufacture. Therefore, a problem is caused in that a large size display device can not be fabricated with these LDCs. On the other hand, the number of drive lines of the cholesteric LCD is not limited, because rewrite and refresh are carried out only when display is to be changed, and the display is held due to the memory characteristic of the LCD once it has been written. The cholesteric LCD, however has a problem such that rewriting requires excessive time.
The current cholesteric LCD necessitates more than 10 seconds to rewrite 1000 lines in the display panel. On the other hand, a page size application such as an electronic book requires less than one second for rewriting one page so as to match the time required to turn over one page of a book manually.
To this requirement, U.S. Pat. No. 5,748,277 “Dynamic drive method and Apparatus for a bistable liquid crystal display” discloses a method for rewriting a passive matrix LCD within one second, the display using cholesteric liquid crystal. The method intends to increase the rewriting speed of the display panel by utilizing a dynamic drive method and a pipeline scheme, the dynamic drive method utilizing a series of stages to control the transition of liquid crystal textures. Such a high speed rewriting scheme allows a display panel using cholesteric liquid crystal material to be used in a passive matrix drive method (i.e. a simple matrix drive method) having an addressing speed more than 1000 lines/second.
FIG. 1 shows an electronic book 10 disclosed in the U.S. Pat. No. 5,748,277. The electronic book comprises a display screen 12, a page selection switch 14, and a memory card or floppy disk 16 which can carry the information to be viewed.
FIG. 2 shows the structure of a liquid crystal panel using a passive matrix drive method disclosed in the above-described U.S. Patent. The structure thereof comprises glass plates 20 and 22, row electrodes 24, and column electrodes 26. Cholesteric liquid crystal is sandwiched between two glass substrates 20 and 22.
Picture elements are formed between opposite row and column electrodes which selectively activate the picture elements. Such activation causes the liquid crystal to exhibit various liquid crystal textures in response to different conditions of electrical fields applied thereto. The liquid crystal assumes the homeotropic texture at a higher voltage. The twisted planar texture and focal conic texture may be stable in the absence of an electric field. The transient twisted planar texture occurs when an applied electric field holding the liquid crystal in the homeotropic texture is suddenly reduced or removed. This state is transient to either the twisted planar or focal conic texture. The liquid crystal of twisted planar state reflects light in the visible spectrum depending on the pitch length of the material to allow the display of white color. The homeotropic state and focal conic state show a weak scattering condition or a transparent condition. If the back side of a picture element is colored in black, the picture element is displayed in black for the homeotropic state and focal conic state. Also, a full-color display may be implemented by stacking display layers, each of these layers reflecting red, green, or blue light. Gradation display may be realized in a cholesteric liquid crystal display panel due to a gray scale characteristic obtained by selecting a voltage and/or time duration the voltage is applied.
In accordance with a dynamic drive method, the cholesteric liquid crystal picture elements are activated in a series of steps to control their transitions during the refresh or update of the display screen. These steps include three active stages and one non-active stage, three active stages consisting of a preparation stage, selection stage, and evolution stage. The non-active stage exists before the preparation stage and behind the evolution stage, respectively. The non-active stage before the preparation stage does not transform the liquid crystal texture. The dynamic drive method using three active stages is referred to as a three-stage scheme.
The preparation stage transforms the liquid crystal to a homeotropic state. The selection stage selects either the maintaining of a homeotropic state or the transformation to a transient twisted planar state. The evolution stage evolves the liquid crystal selected so as to be transformed to the transient twisted planar state during the selection step to a focal conic state, and holds the homeotropic state of the liquid crystal selected to remain in the homeotropic state during selection stage. The final non-active stage maintains the focal conic state as it is, and transforms the homeotropic state to a stable twisted planar state.
A four-stage scheme may be implemented by adding a pre-selection stage behind the preparation stage, the pre-selection stage allowing the liquid crystal to relax to a transient twisted planar state. Adding the pre-selection stage may increase the speed for activating the picture elements.
In the drive method using a series of stages, the determination of a final liquid crystal texture of a picture element depends upon the voltage applied to the electrodes during the selection stage, with the applied voltages during other stages being the same. All of the picture elements, therefore, require the same non-active voltage, the same preparation voltage, and the same evolution voltage, so that the time may be shared during the non-active stage, preparation stage, and evolution stage by employing a pipeline argorithm. Accordingly, a plurality of electrodes may be addressed at the same time by a non-active voltage, preparation voltage, and evolution voltage.
In the above-described U.S. Patent, while applied voltages to the row electrodes and column electrodes have a vibrating bipolar square waveform, respectively, it is known that a vibrating unipolar square waveform may be used by selecting the magnitude of applied voltage and the time duration of applied voltage. Using a unipolar square waveform results in the decrease of a swing width of voltage applied to a display driver and the cost reduction of the driver. Whether the applied voltage is bipolar voltage or unipolar voltage, the voltage applied to a picture element, i.e. the voltage difference between the voltages applied to a row electrode and column electrode is a bipolar voltage. Such bipolar voltage applied to a picture element is referred to as an alternating voltage hereinafter. The reason why an alternating voltage is used is to decrease the effect of impurities dissolved in liquid crystal material and expand the life time of the liquid crystal material.